The invention will primarily be described in relation to corrugating medium used in making corrugated board as used in the manufacture of, for example corrugated containers.
As is well known, it is very important to maintain consistently well formed flutes to ensure the proper adhesion of single face and double face liners to ensure the board produced will have the proper strength and aesthetic characteristics.
It will be apparent that it is important to be able to understand and/or analyze the flute profile, particularly, flute height, on-line during operation so that immediate steps may be taken to rectify any significant problems that are detected. It is the height of the flute and/or variation in height of the flute that determines many of the final characteristics of the board.
A method for off-line inspection of flute profile, particularly variation in flute height is simply an examination of the produced board visually for defects such as high-low flutes and caliper. These measurements are subjective and are made on the final combined board leaving the line. Thus variation of the flute profile of the corrugated medium in the single face is masked by the combination with the double face liner.
It is also known to slit the board across the corrugations, i.e. through one liner and the corrugated medium and fold the board back on itself by creasing the opposite liner and then visually compare the successive flutes so exposed.
Certain off-line tests may also be used such as iodine stain applied to the glue line after separating the liner from the medium to show irregularity in the glue line, strength measurements on the combined box board, overall appearance (e.g. blisters) and finished print quality.
More recently, on-line devices have been attempted, for example devices contacting the fluted medium exiting the corrugator roll to roll nip have been used in lab corrugators to try to determine flute heights and variations thereof.
One technique, known to the trade is the "CTP" technique wherein a collimated light beam is passed tangentially across the flute tips of single face passing over a roller. This technique has been used on a laboratory corrugator and is not particularly suitable for high speed operations since the single face must be bent around a relatively small diameter roll in order that the individual flute tips can interrupt the collimated light beam. This device also gives no indication of flute depth and it is possible that medium surface roughness could effect the results. This device is described by Schoene, E. and Serre, J., "French Development Provides Continuous High/Low Measurement", Boxboard Containers, p 35-46 (Oct. 1983).
Another technique known as the IPC technique is suitable for slow, detailed analysis of flute profile variations which permits precise measurement of variation from point to point along the profile. A laser beam is projected at a first angle toward the flutes of the corrugated medium and the light reflected at an acute angle to the laser beam is collected. This device to date is limited to relatively low speed laboratory testing and has not been used commercially on-line due to stringent mounting requirements.